In a case where food is packed with a packaging material and stored, food deterioration is often accelerated by a substance penetrating from the exterior of the packed body. In order to prevent food from being deteriorated by a substance penetrated from the exterior, methods are conventionally adopted in which food is packed with a packaging material having low substance permeability, in particular, gas permeability. Typical examples of such substance that penetrates from the exterior and accelerates food deterioration include oxygen. In this case, methods are adopted, for example, in which food is packed with a packaging material having low oxygen permeability, and moreover, an oxygen absorbent is enclosed within the packaging container or the like.
In a case where methods are adopted in which a substance is prevented from penetrating from the exterior into food or in which the substance is absorbed, a means for detecting the substance that has penetrated from the exterior and judging the influence of the substance on the food is necessary in order to determine the effectiveness of these methods. Mechanical measurement methods are accurate, but more simply, methods using a color reaction of indicators are used. Various detection methods have been conventionally proposed in particular for oxygen.
For example, Japanese Laid-Open Patent Publication No. 54-48294 discloses an oxygen detecting composition comprising methylene blue and a reducing agent in an amount that can change the methylene blue to substantially colorless in the presence of water, wherein the oxygen detecting composition is colored upon detection of oxygen. Japanese Laid-Open Patent Publication No. 8-118551 discloses a composition consisting of methylene blue, hydrosulfite (Na2S2O4), agar, and water, and a method for detecting oxygen penetrated into a container using the composition. Japanese Laid-Open Patent Publication No. 2005-91008 discloses an oxygen indicator in which a redox indicator is contained in various supports.
Penetration of oxygen is detected, for example, by enclosing such oxygen detection means within the interior of a packed body of various types of food, when selling the food. However, for example, assuming that food such as meat or ham having a certain size and shape is packed, most of such means in these states are often insufficient for checking the barrier property of the packaging material per se, or the barrier property of a bag or container made of the packaging material, under suitable conditions for handling the food.
For example, in a method for checking penetration of oxygen based on a color change of methylene blue by placing an aqueous solution containing methylene blue and a reducing agent in a packaging material in a predetermined form such as a container or bag, it is possible to judge whether or not as a whole, oxygen has penetrated, but it is impossible to judge from which portion of the container or bag, the oxygen has penetrated. The reason for this is that the aqueous solution flows and convectively circulates in the interior of the container or bag. In particular, in a case where the shape of the packaging material is changed according to the shape of the food, or in a case where the packaging material is subjected to external treatment such as heat treatment depending on the treatment conditions for food, it is difficult to judge the barrier property or the like of the packaging material.
As disclosed in Japanese Laid-Open Patent Publication No. 54-48294, it is possible to prevent an aqueous solution containing methylene blue and a reducing agent from flowing or convectively circulating, by impregnating a support such as paper or film with the aqueous solution, or by printing the aqueous solution onto paper. However, in both of these prevention methods, since the detection means are realized in the form of a film, it is impossible to test the barrier property of packaging materials under conditions that match various forms of food or various types of treatment necessary for food. For example, even though it is possible to detect oxygen penetrating into the packed body, it is impossible to detect whether the oxygen has penetrated through the entire packaging material or the oxygen has penetrated through a pinhole that has been present in the packaging material.